This invention relates to a continuous process for producing a pressed-wood composite product from a prepared pre-assembly mat which includes selected wood components along with intercomponent, heat-curable adhesive. In particular, it relates to such a method, and also to an apparatus for implementing this method, which utilizes time-spaced stages of both pressure application and heat introduction as an approach for achieving the final integrated pressed product. Such an approach yields a superior compressed-wood product, does so with equipment which is compactly and efficiently organized, and accomplishes processing in steps which offer significant control over end-product results.
A typical end product resulting from practice of the present invention might, for example be plywood, or laminated veneer lumber (LVL), which, after production can be cut for use, or otherwise employed, in various ways as wood-based building components. The starter material, so-to-speak, which can be effectively treated by the process and the machinery of this invention, insofar as the relevant wood componentry is involved, would typically be, in addition to a suitable heat-curable adhesive, (a) thin sheet veneers of solid wood, (b) oriented strands (or other fibrous make-ups) of smaller wood components, (c) solid wood lumber of various sizes, (d) already pre-made expanses of plywood which themselves are made up of thinner layers of wood plys, or (e) other wood elements.
Describing for a moment conventional LVL fabrication processing, LVL is typically made of glued, thin, veneer sheets of natural wood, utilizing adhesives that are mostly formed of Phenol Formaldehyde formulations which require heat to complete a curing process or reaction. In the state of the art today, there are several well-known and widely practiced methods of manufacturing and processing to create LVL. The most common pressing technology involves a platen press, and a method utilizing such a press is described in U.S. Pat. No. 4,638,843. Pressing and heating is typically accomplished by placing precursor LVL between suitable heavy metal platens. These platens, and their facially "jacketed" wood-component charges, are then placed under pressure, and are heated with hot oil or steam to implement the fabrication process. Heat from the platens is slowly transferred through the wood composite product, the product shrinks and compresses under pressure to the desired final thickness, and the adhesive cures after an appropriate span of pressure/heating time. This process is relatively slow, often taking, with conventional equipment of the type generally just described, about 19-minutes or so (per unit area) to compress and cure a finished product having a final thickness of about 1.5-inches.
Recognized today in the art is the fact that the addition of suitable radiofrequency (RF) energy to the environment within (i.e., in between) opposing press platens can accelerate the heating and curing process. Accordingly, the use of this augmentive RF approach to heating can shorten fabrication times. However, there are occasions involving problems with arcing due to high voltage that is in existence with respect to such RF energy employment. Such arcing is typically exacerbated by the presence of uncured and moist adhesive which squeezes out to regions of exposure on the sides of the material being pressed. U.S. Pat. No. 5,628,860 describes an environment where this kind of situation can occur.
Another conventional process employed for the preparation of LVL is described in U.S. Pat. No. 5,895,546. This patent discusses the use of microwave energy to preheat loose LVL lay-up materials, which are then finished in a process employing a hot-oil-heated, continuous-belt press. This process avoids the RF arcing problem just mentioned above by the fact that it typically employs a significantly lower-voltage and a higher-frequency heating energy than that which is employed in an RF environment of the traditional approach. However, this type of processing still requires conventional hot-oil energy in the final pressing stage of activities. For example, a press time for the production of a final 1.5-inches product is typically here around 11- or 12-minutes (per unit area). For a much thicker product, for example, for a final LVL product with a thickness of about 3.5-inches, production time can be three or four times this length. Further, a problem often specifically associated with microwave pre-heating is that such pre-heating is carried out on what can be referred to as loose lay-up (pre-assembly) materials, and any line stoppage can cause adhesive to dry out and become unusable for completing product production. Further, in any situation wherein a belt press is employed, such a press is a very expensive piece of equipment, much more expensive than a platen press, and consequently, not always the most desirable machinery-route (economically) to use.
U.S. Pat. Nos. 4,456,498 and 5,228,947 disclose processes utilizing microwave energy during the adhesive curing and compression process. Such energy is applied through ceramic-covered wave-guides that are positioned in openings between continuous-belt press sections in formation machinery. This approach to production is typically limited to the production of relatively large beam materials, and thus does not have a very wide-ranging applicability. Additionally, it typically requires a higher than often desirable spread of glue, and a significant wood densification--matters which are not always particularly wanted.
In this setting, a general object of the present invention is to provide a unique, continuous-flow process, and a system for implementing the same, which offers a wide degree of versatility with respect to the fabrication of a pressed-wood composite product, utilizing extremely efficient machinery which is relatively inexpensive in comparison with prior art machinery, and which can accomplish complete fabrication and adhesive curing with a relatively low expenditure of energy, in a relatively short period of time, and with substantial adjustable control afforded over processing parameters (pressure, temperature, time) in any given "processing window" for each region in processed material. The term "processing window" is here employed to refer to the overall time during which each region in the material that is being processed is subject to the different, required processing activities. By establishing, selectively, the physical space occupied (in the system of this invention) by each processing component, continuous-flow processing is enabled in a setting where greater or lesser processing times for exposure to the specified activities furnished by any given component can be varied simply by charging/adjusting/designing the physical size of that component, as measured in the direction of material travel in the system. In addition, and very significantly, the process and system of the present invention can, in most instances, produce a resulting product which is superior to its prior art counterparts in terms of economy of manufacture, stability in final form, and ease of confident usability either as an end product, or as a precursor to yet another, future end product.
According to a preferred manner of practicing the invention, a prepared mat of preassembled wood components, and intercomponent distributions of an appropriate heat-curable adhesive, are fed in a continuous-flow manner through a processing zone wherein the mat is subjected to time-spaced intervals of compression pressure, along with time-spaced intervals of microwave-introduced heat. While, within the context of the generally unique concept of this invention involving employing such "time-spaced" activities, the specific organization of pressure and heating intervals is a matter of wide and free choice, one approach which has been found to be extremely successful in the making of, for example, LVL, is an approach which utilizes a "cyclic" application of pressure, i.e., cycles alternating between high and lower pressure as material travels through the processing zone, interspersed with "cyclic" intervals of heat introduction promoted by the use of microwave radiation which is introduced to traveling material in the regions between where high pressure is applied to the traveling material. Thus, a preferred embodiment of a system which implements this approach is one wherein a prepared mat, including selected wood components and selected intercomponent heat-curable adhesive, is fed into a region where this mat is held between suitable facial-pressure-applying "sheets" of material, such as traveling, microwave-transparent, thick platens. This overall sandwich assembly, as such travels through the processing zone created in accordance with the invention, is subjected to recurrent, intermittent cycles of the high and low pressure created by the passage of the assembly between successive adjacent pairs of adjustable pinch rolls. Microwave radiation units are placed in the regions intermediate adjacent pairs of pinch rolls, and there act to create a staged (or stepped) kind of heat build-up during the travel moments when "sandwich portions" pass from one set of pinch rolls to the next-adjacent set of such rolls. The pressure-conveying platen additionally act as a heat jackets that contribute to maintaining internal processing heat in material passing through the system.
Another approach, which very similar to the one that has just generally been described, is one wherein the mentioned microwave-transparent platens (or the like) that travel with the mat of to-be-compressed material are replaced by two, continuous, elongate, spaced and opposing jointed/scarfed sheets of wood veneer which act in the places of individual pressure platens, and which become incorporated ultimately in the finally produced LVL product.
As will become apparent, the exact organization of components used to apply pressure, and to introduce microwave heating energy, can be determined and adjusted to suit different particular fabrication requirements. But preferably, these elements according to the invention, are spaced and interspersed with one another in a kind of alternating fashion, whereby what can be thought of as the peaks of compression pressure, insofar as traveling material is concerned, are bridged by lower pressure moments that are filled with the application of microwave heating energy. Also, and preferably, that heating energy functions in a kind of "stair-step" fashion to build up the internal temperature in the forming material as such travels through the processing zone. A preferred organization of pressure-application pinch rolls, and of microwave radiators, is described herein, as are also certain modified arrangements which have been found to be quite useful in certain instances.
Another aspect of the present invention contemplates the formation of LVL and like products, and machinery for accomplishing such formation, wherein the mat of composition material which enters the processing zone mentioned above is subjected preliminarily to a stage of initial compression pressure and heating to prepare it (in a slightly different fashion) for entry into that zone. Such a modification is illustrated in one of the drawing figures herein, and is described in the text below. Other modifications are also illustrated and described.
The various features, objects and advantages that are offered and attained by the present invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.